TRENTON, Dec. 13 — With New Jersey’s Legislature set to vote Thursday to establish civil unions rather than same-sex marriages, it remains unclear whether this approach can actually fulfill the mandate from the state’s Supreme Court to guarantee the same rights and benefits for gay and straight couples.

The New Jersey State Bar Association is one of several organizations that have criticized the proposed civil union law, saying in a statement that it “will create a separate, unequal and unnecessarily complex legal scheme” that does not satisfy the Supreme Court’s directive.

Advocates on both sides of the gay marriage debate are uneasy about the legislation, which was introduced just nine days ago and has seen little scrutiny, with only two committee hearings before the scheduled floor debate. And among gay couples who plan to obtain civil unions, as well as lawyers who work on civil rights issues, questions about a new, parallel institution are piling up.

The most likely problems, they say, will arise beyond the state’s borders — beyond the reach of its Supreme Court and the Legislature. The biggest among many unknowns is how New York will treat the unions, no small question in New Jersey, which has 335,000 residents who work in New York. (An additional 117,000 work in Pennsylvania.)

The answer seemed clear when the New York Attorney General’s Office said in an advisory opinion in 2004 that New York should recognize a same-sex marriage “or its legal equivalent” from another state.

But last fall, in a case involving a gay couple from New York who had entered a civil union in Vermont, a New York appellate court refused to recognize the surviving partner as a spouse for the purpose of filing a wrongful death suit. The court pointed out that the Vermont Legislature created a separate institution rather than allow gays and lesbians to marry, implying the couples can be treated differently.

Spokesmen for the New York attorney general, Eliot Spitzer, who will become governor in January, said they could not expand on the 2004 advisory opinion or comment on the New Jersey law until it is in place.

Most states recognize marriages legally performed in another state even if they would otherwise be invalid — for example, ages of consent differ across state lines. But the federal government, and many states outside the Northeast, have enacted exceptions refusing to recognize same-sex marriages and by implication, legal experts say, their civil unions. (For federal benefits, like taxes, civil unions and marriages are indeed treated equally: same-sex couples are denied benefits no matter their status.)

The few precedents available in New York involve marriages performed in Massachusetts, the only state where gay and lesbian couples can marry, and civil unions from Vermont or Connecticut.

“It’s clear in New York that marriages validly performed in another state should be respected,” said David Buckel of the Lambda Legal Defense and Education Fund, which represented the plaintiffs in the original New Jersey case that spawned the Supreme Court decision nine weeks ago.

Mr. Buckel said that many New York employers, insurers and local governments did recognize same-sex marriages from Massachusetts, and the state comptroller has said the state would recognize same-sex marriages for the purposes of pension benefits. “But,” he said, “it’s less clear that this will be the case with civil unions.”

“We’re going to fight for New Jersey couples,” he said. “We’re going to have to fight a lot more than we would have if they were married and had that security.”

William C. Duncan, the director of the Marriage Law Foundation, a Utah-based legal organization that opposes gay marriage, said that “there are bound to be significant interstate questions because civil unions is a novel idea compared to what states have been doing for decades.”

With same-sex marriages, “couples could go to another state and say they want their marriage recognized,” Mr. Duncan said. “At least there, the default rules apply. With civil unions it’s really all up in the air.”

Many New York residents may seek civil unions in New Jersey as well, since the law contains no residency requirement.

“If you’re a gay person living on Christopher Street, you can take a three-minute PATH ride to Jersey City” for a civil union license and ceremony, pointed out Steven Goldstein, the chairman of Garden State Equality, the group that has led the campaign for same-sex marriage. “Then you go home. Well, test it on your tax forms on April 15 and see what happens.”

Alan Van Capelle, the director of Empire State Pride Agenda, a gay-rights group in Manhattan, noted that New Yorkers are already able to obtain civil unions in Connecticut and Vermont, a relatively short drive away. “So now, it’s which do you like better? Do you like Foxwoods or do you like Atlantic City?” Mr. Capelle said.

In Pennsylvania, there is even less likelihood that civil union couples would be treated like married couples, since that state bars recognition of same-sex marriages (it has no guidelines for civil unions).

Within New Jersey, the new law demands that civil union couples be treated like married couples by private employers, hospitals, social service organizations and companies like insurers that do business in the state. The legislation spells out the benefits to be extended, like family leave and full adoption rights, the right to change a surname without a court petition, workers compensation benefits and protections under the laws governing health benefits and pensions.

Still, Mr. Goldstein said, it might be harder to enforce these rights without the word “marriage” attached.

“You get on paper all 700-plus rights that marriage provides,” he said. “You get those on paper. We’re just so scared that these rights won’t come to life in the real world.”

Many advocates of same-sex marriage say they worry that the benefits afforded by civil unions — like the more modest package of benefits already available through New Jersey’s 2004 domestic partnership law — would not be easily understood.

Mr. Buckel recalled a plea from a woman in South Jersey whose partner was hospitalized and unconscious. Hospital staff members, she reported, had removed her partner’s commitment ring and refused to give it to her, and then insisted that they could deal only with blood relatives.

New Jersey’s domestic partnership law allowed the woman to make medical decisions for her partner, but only after the Lambda Legal staff called the hospital’s lawyer, Mr. Buckel said, was she able to do so.

A version of this article appears in print on , on page B1 of the New York edition with the headline: Doubts Persist As New Jersey Moves Toward Civil Unions. Order Reprints|Today's Paper|Subscribe